Aggregate mixing apparatus having spherical batch mixing vessel with jet pump to help load material and single pneumatic source to pressurize mixing vessel and drive jet pump

ABSTRACT

Mixing apparatus includes a batch mixing vessel with an inlet to receive constituent material, an agitation assembly for mixing the constituents and an outlet to convey batches of the mixed is provided. The apparatus includes a pressurizing device for pressurizing the vessel to assist in evacuation of the mix, and a conduit through which the mix is conveyed. A single pneumatic source is used both to pressurize the mixing vessel and to drive a jet pump including the batch loader. During the mixing cycle, the compressor used to discharge the finished mix is generally idle. This compressor can be used during these periods to drive a jet pump to load the mixing vessel. Therefore, manual loading is avoided and can be achieved such that the mixing cycle time is not interrupted.

BACKGROUND OF THE INVENTION

The present invention relates to mixing apparatus, for mixing materials,such as aggregates used in the construction industry, and in particularto apparatus which not only mixes the materials, but then conveys themixed material to the required point of discharge. Such apparatus iscommonly known as mixer-placer apparatus. The invention also relates toa mixing vessel and to a suction conduit assembly for such apparatus.

Concrete floors, for example, may be laid and screed with mixer-placerapparatus. Although traditionally, this technology is used in theconstruction industry, the technology can be applied to other industrieswhere mixing and displacement of materials is required.

Existing mixer-place machines comprise a diesel engine which drives anair compressor. The diesel-engined compressor is typically mounted upona wheeled chassis, or road trailer, which also supports a cylindricalbatch holding vessel. The holding vessel serves as a mixing vessel andthus includes a rotatable mixing paddle arrangement. The mixing paddlearrangement is powered either by a hydraulic pump and motor or a beltdrive or gear box. The cylinder is mounted with its axis horizontal andthe mixer paddle drive shaft is on the axis of the cylinder.

The holding/mixing vessel is provided with a large charging opening inthe upper part of the cylindrical side wall, the opening being closableby a lid and seal arrangement, which provides an air-tight seal to thevessel on closure of lid. This allows the contents of the vessel to bepressurised.

A delivery hose is connected to an outlet provided in the lower sidewall of one end of the mixing vessel, to deliver the mixed product tothe required location once the mixing stage is completed. A pneumaticsupply line from the compressor is connected to the mixing vessel toeffect discharge of the mixture, once mixing of the constituents iscompleted. Therefore, the principal function of the compressor is topressurise the mixing vessel for discharge.

In use, when the mixing apparatus is to be used for mixing a floorscreed material, the mixing vessel must first be charged with the sand,cement and water, in the required proportions. The lid covering thecharging opening is removed, and a measured quantity of sand isshovelled into the mixer vessel from an adjacent sand heap. A bag ofcement is split and also emptied in, together with the required amountof water and any additives required. During this process, the paddlesare rotated, and once loaded, the lid fastened down securely, whilst themixing process is completed.

Once mixing is complete, compressed air is admitted to the mixing vesselvia the air inlet. The paddles in the mixing vessel urge the materialtowards the discharge outlet. As the mixture covers the opening, themixture discharges and is conveyed by the pneumatic pressure from thecompressed air through the delivery hose. A hose tripod device may beemployed to kill the energy in the concrete and allows it to fall in aheap beneath it whence it is spread and levelled.

It will be appreciated that the time spent filling the mixing vesseloccupies a significant part of the cycle, and that this is alabour-intensive stage.

In addition, with current mixer-place machines, great care needs to betaken to clean the lid for the charging aperture, and its seal to ensurethat the seal is effective. This step is time consuming, and adds to theoverall cycle time for a batch of mixture. In addition, it has beenfound that the handling at this step can serve to accelerate thedeterioration of the seal.

If the total cycle time could be reduced, and if the labourer's timecould be better utilized, then the total operational efficiency of theprocess can be improved.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide mixing apparatuswhich attempts to address one or more of the disadvantages outlinedabove, and/or to provide improvements generally.

According to one aspect of the present invention there is providedmixing apparatus comprising a batch mixing vessel having an inlet toreceive constituent material, an agitation assembly for mixing theconstituents within the vessel and an outlet to convey batches of mixedmaterial; the apparatus further comprising means for pressurising themixing vessel to assist in evacuation of the mixed constituents from theoutlet, and a batch loading means including a conduit through which theconstituents are conveyed in use by fluid flow means; wherein a singlepneumatic source is used both to pressurize the mixing vessel and todrive a jet pump comprising the batch loading means.

According to a second aspect of the invention, there is provided amixing vessel for mixing apparatus comprising a sealable charging inletfor loading constituents to be mixed, and an outlet for discharging themixed constituents, and means for pressurising the vessel; wherein themixing vessel is generally spherical.

In yet a further aspect, there is provided mixing apparatus comprising amixing vessel according to the second aspect of the present invention.

According to the present invention, it has been identified that duringthe mixing cycle, the compressor, used to discharge the finished mixtureis generally idle, and required only at the time that the mixture is tobe discharged. By utilising the compressor during conventionally idleperiods, it has been found that the same compressor can also be used todrive a jet pump. Such a jet pump can be used to create sufficientvacuum to load the mixing vessel with the mixture constituents, withoutthe need for manual loading, and within a sufficient time frame whichwould still allow the required mixing cycle time to be achieved.

The compressor is sized to produce the required discharge of screen mixto the required distance and elevation, but is only needed for a shortpart of the cycle for that purpose. By making use of the existing powersource and compressor to provide vacuum suction on the mixing vesselinput, a reduced loading time, and thus batch time, is achieved, and theloading itself is far less labour intensive.

It has been possible to design a jet pump to use an air volume andpressure to generate a vacuum in the mixing vessel with the cycle timeachievable by traditional means (men shoveling) which is identical tothat necessary for the required discharge flow rates and pressure.

The batch loading means comprises a conduit. A further aspect of theinvention relates to the suction conduit. In view of the stiffness ofconduit hose required to load the constituent materials, handling suchhose can be difficult.

In a further aspect of the invention, an inlet suction conduit assemblyis provided which includes adjustment means, such that the effectivelength of the conduit assembly can be varied in use, in a controlledmanner. Preferably, the adjustment means comprises a section which iscapable of telescopic movement, and this may be achieved by hydraulicactuation. It is preferred that the suction conduit assembly isrotatably mounted. In a further embodiment, the suction conduit isarticulated. In such an arrangement, the articulated pipe is providedwith a gimbal joint at the connection to the vessel and a knuckle jointwith the horizontal axis part way along its length.

In a further preferred embodiment, the suction conduit assembly isprovided with means operable to vibrate the intake, to facilitate entryof the constituent material, such as sand, into the suction inlet.

It has been identified that in order to create a sufficient vacuum toload the mixture constituents in time required, an optimum diameter ofsuction pipe or hose of the suction conduit assembly can be determinedby test guided by calculation.

It is preferred that the suction conduit assembly is provided with somebalancing mechanism, and is also provided with the ability to swinghorizontally and vertically so that its suction inlet could map anythree-dimensional volume within its reach. In an embodiment, sensormeans are provided to enable the apparatus to consume a heap of mixtureconstituents systematically and without human intervention. In anembodiment, such a sensor may include a timer, for timing the length oftime that the compressor has been operating, the quantity of materialcollected being proportional to the length of time of operation.

In an embodiment there is provided a hydraulically operated telescopicsuction pipe for the suction conduit assembly. The power assistance isprovided to the telescoping action.

In an embodiment, a short gimbal support is provided, allowing swingingof the pipe within a solid angle of +/−30°. The hose is provided withopen convolutions which are non-clogging. The pipe is partly supportedby a gas spring, the degree of support depending upon the extension andamount of sand within it. The gimbal arrangement allows hydraulic powerof both swinging and raising and lowering, using circuit design andcomponents identical with those which are used for its extension andretraction.

The end of the inlet pipe may be provided with a whirling ball vibratorto motivate the sand, and an alternative would be to mount the vibratorelastically, with probes to motivate the sand.

In one aspect of the present invention it is preferred that the mixingvessel of the mixing apparatus is generally spherical. It has been foundthat by replacing the conventional cylindrical shape of mixing vesselwith a generally spherical vessel has a number of technical advantages.

The spherical form has been found to eliminate bending stresses in theshell. This enables a thinner shell material to be used, which requiresonly a single fabrication weld. This can be compared with three or fourweld required in the conventional cylindrical form. This results in acost saving not only on materials, but also reduced costs in the weldingstages, and in the requisite subsequent testing of such welds.

It has also been found that the spherical cavity allows more completefilling (the spherical shape provides a better shaped air space abovethe mixture to reduce interference with the incoming constituents), morecomplete discharge (the shape also provides a positive low-point) andeasier cleaning. The generally spherical shape also provides a morespace-efficient structural form, such that the apparatus occupies lessspace.

In an embodiment, the mixing vessel is provided with a protective liner.The protective liner comprises a plurality of segments. The use of“orange-peel” segments to provide a protective liner for the mixingvessel avoids the use of castings, which would be heavier, more costlyand more liable to fracture. The double curvature facilitatesmanufacture of the lining segments by simple rolling. It has beenidentified that in use, most of the wear takes place towards the lowerend of the “segments”. In view of their shape, the liner segments arereversible, and can simply be rotated through 180° and then re-used.

In an embodiment, the mixing vessel is provided with a level sensor. Thelevel sensor preferably operates on the principle of theoscillating-vane level sensor. When the vessel is under vacuum, apressure difference exists which can be used to energise the incomingwater to form a jet. This jet may be directed against any parts whichwould benefit from washing, such as a screen through which the incomingsolids have to pass (to filter out stones and fragments of cement bagsetc.), the level sensor, and the suction inlet control valve. Thewashing of the seal of the lid is advantageous to reduce itsdeterioration, in view of the abrasive materials being mixed.

In an embodiment, there is provided an hydraulic drive for the mixingpaddles. This is considered advantageous over other drive means, becauseit frees the mixture vessel from constraints of location relative to theengine for the compressor.

In an embodiment, clamping means are provided to prevent accidentallifting of the lid/cover of the mixing vessel in any condition ofinternal pressure of vacuum. The clamp is in an over-centre arrangement,such that high pressure in the vessel prevents operation because anunrealisable operator force is required. At a low vessel pressure,operation of the clamps may be prevented by the additional releasemechanism. Accidental operation of release mechanism is also prevented,and venting of any escaping jet is controlled. The edge of the lid isformed with a downward deflector provided by the profile of the edge ofthe lid, in such a way as to prevent injury to the operator and at thesame time to provide necessary structural integrity. Forcing the releaselever would vent the pressure safely and stop the engine.

Other modifications which are envisaged within the scope of the presentinvention is the use of one or more additional smaller jet pumps whichcan be used to load additives such as fibres to the mixing vessel. Thesewould be driven from the same air supply as the main jet pump butoperated only as long as necessary. In this way, the provision of anadditional or secondary self-loading facility is achieved at almost noadditional cost. The use of additional small jet pumps preventsinteraction of loading pipes or hoses and possible unsolicited variationof mix.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the various aspects of the present invention will now bedescribed, by way of example only, with reference to the followingillustrative drawings in which:

FIG. 1 is a side elevation of a suction conduit assembly in accordanceto one aspect of the present invention, attached to a holding vessel;

FIG. 2 is a plan view of the embodiment shown in FIG. 1 which is in partin cross section;

FIG. 3 is an enlarged view of FIG. 2; and

FIG. 3A is an enlarged view of the area circled in FIG. 3;

FIG. 4 is a schematic diagram of a mixer-placer machine according to asecond aspect of the invention;

FIG. 5 is a detailed view of a mixing vessel according to a furtheraspect of the invention;

FIG. 6 a is a partial cross section of the lid/seal arrangement for themixing vessel of FIG. 5 taken along line 6 a-6 a of FIG. 6 b, and FIG. 6b is a plan view of FIG. 6 a viewed along arrow 6 b of FIG. 6 a;

FIG. 7 a is a plan view of the lid housing for the mixing vessel, andFIG. 7 b is a sectional view on line 7 b-7 b of FIG. 7 a showing theinlet screen;

FIG. 8 a is a plan view of the mixing vessel lid and FIGS. 8 b, 8 c, 8d, 8 e, 8 f, 8 g and 8 h are further cross-sectional views of the lidlocking assembly;

FIG. 9 is a cross section view of a pressure release valve assembly forpart of the lid closure mechanism of FIGS. 8 a to 8 h;

FIG. 10 a depicts a modification to a standard ball valve for thesuction pipe 160, and FIG. 10 b is a sectional view of FIG. 10 a alongline 10 b-10 b;

FIG. 11 illustrates the valve assembly provided in the discharge hose;

FIG. 12 is a diagrammatic view of the drive shaft assembly of theapparatus;

FIG. 13 is a side elevation of a suction conduit assembly according to afurther embodiment of the invention; and

FIG. 14 is a partial plan view of the embodiment of FIG. 13.

DETAILED DESCRIPTION

Referring to FIG. 1, a suction conduit assembly 1 through which materialis conveyed comprises a first pipe section 3, a second pipe section 4and an intake 5. The bore of the first pipe section 3 is dimensionedsuch that the second pipe 4 is capable of telescopic movement withinsaid first pipe section 3, so that the effective length of the suctionconduit can be varied.

Referring to FIGS. 3 and 3A, attached to the inner surface 12 of thedistal end 7 of second pipe section 4 is a sealing skirt 6. The sealingskirt 6 is of substantially frusto-conical shape and forms a substantialseal with the inner surface 10 of the first pipe section 3.

Similarly, a sealing skirt 9 is provided on the outer surface 13 of thedistal end 8 of the first pipe section 3. The sealing skirt 9 is ofsubstantially frusto-conical shape and forms a substantial seal with theouter surface 11 of the second pipe section 4. The distal end 7 and thedistal end 8 are flared and tapered respectively to ensure sealingcontact of the skirts with the respective surfaces.

The suction conduit assembly 1 further comprises pistons 14 a and 14 band associated hydraulic cylinders 15 a and 15 b. The pistons 14 a and14 b are secured to the second pipe section 4 in the following manner.Two pairs of plates 16 and 17 (one plate of each pair is shown in FIGS.2 and 3) which are rigidly attached to the second pipe, are spaced so asto accommodate bored portions 18 a and 18 b. Bores 19 a and 19 b arealigned with holes in the plate pairs 16 and 17 so that securing pins(not shown) can pass there through. The bored portions 18 a and 18 b areattached to the respective pistons by ball joints (not shown). Thepistons 14 a and 14 b are actuated by respective hydraulic lines 24, thehydraulic lines being in communication with opposite ends of eachcylinder 15 a and 15 b. The pistons 14 a and 14 b may conveniently bepowered from the hydraulic system associated with the holding vessel 45,if one is so provided.

Each of the hydraulic cylinders 15 a and 15 b is attached at twolocations on the first pipe section 3. The hydraulic cylinders aresecured in the first instance by securing pins (not shown) through platepairs 20 and 21 (one plate of each pair is shown in FIGS. 2 and 3) andbored portions 22 a and 22 b are attached to the respective cylinders byball joints (not shown).

A gimbal 44 comprising two double-limbed components 26 and 27 and frame32 is provided between the first pipe section 3 and pipe 37, thedouble-limbed components 26 and 27 partially surrounding a length offlexible hosing 25. The double-limbed component 26 is attached to frame32 for rotation of the pipe sections 3 and 4 about an axis Y-Y. Thedouble-limbed component 27 is attached to the frame 32 in a planeperpendicular to the axis Y-Y for rotation of the pipe sections 3 and 4about an axis X-X. The pipe 27 is clamped to pipe 38 by a Bauer clamp39, and the pipe 38 leads up to a shut-off valve 40 and then into aholding vessel 45. The holding vessel 45 may house agitators for mixingmaterials therein.

A vibrator 47 is provided on the second pipe section 4 and comprises acircular track 48 around which a ball bearing is circulated by means ofa compressed air supply (not shown). A support rod 60 is providedbetween the underside of the suction conduit assembly 1 and the holdingvessel 45.

In use, the assembly operate as follows. Evacuation means 58 comprisesan air outlet 61, an evacuation pump 62 and an isolation valve 63. In anembodiment, the evacuation pump 62 is a jet pump, as it is notpracticable to protect the pump with certainty from solid particles. Thejet pump is preferably of the type sold under the trade mark GENFLO™. Ifthe holding vessel 45 is part of the apparatus for pneumaticallyconveyed batches of mixed material, then the jet pump may convenientlybe driven from the air compressor used therefor.

After the shut-off valve has been opened and the evacuation pump hasbeen started, an operator then directs the intake 5 of the second pipesection 4 at a heap of material (not shown) to be loaded into theholding vessel 45. The operator then, via suitable controls, extends thesecond pipe section 4 to penetrate the heap of material, and in sodoing, material is taken into the conduit assembly 1 and then into theholding vessel 45.

Once the second pipe section 4 has been fully extended, said second pipe4 section is then retracted out of the heap after which the second pipe4 is again extended into the heap to load further material into theholding vessel 45. This repeated extension and retraction of the secondpipe section 4 thus produces a reciprocating action, the rate of whichis determined by the characteristics of the material to be loaded. Thevibrator 47 produces an orbital motion of the intake 5, thus ensuringthat the intake 5 does not clog, and that any clumps of material aroundthe intake 5 in the heap are loosened.

If the holding apparatus 45 is part of apparatus to convey batches ofmixed material pneumatically, then once the required amount of materialhas been loaded into the vessel then valves 40 and 63 can be turned offand an outlet valve (not shown) opened. The holding vessel can then bepressurised to force the mixed material out of the vessel.

Handles 30 mounted on the pipe sections 3 and 4 ensure that once thesuction conduit assembly is detached by the clamp 39, it can be carriedwith a reduced risk of causing damage.

In a modification of the assembly 1, the gimbal 44 may be adapted to beused with or replaced with appropriate service means so as to controlthe rotational movement of the suction conduit assembly.

In another modification of this aspect of the invention, the second pipesection 4 may be provided with an attachment to facilitate the loadingprocess of a given material. The attachment may, for example, comprisean actuated scoop to capture material in the heap to be sucked awaythrough the intake 5.

FIG. 4 is a schematic diagram of a mixer-placer machine according to asecond aspect of the invention. As can be seen, the apparatus generallydesignated 100 comprises a diesel engine 110 which drives an aircompressor 120. The diesel engine 110/compressor 120 is mounted upon awheeled road trailer 130, which also supports a generally sphericalbatch holding, or mixing, vessel 140. The mixing vessel 140 includes arotatable paddle assembly which is mounted on a drive shaft powered byan hydraulic pump and motor assembly 150 which is driven by the dieselengine 110.

The top of the mixing vessel 140 is provided with a closeable charginginlet into which the materials to be mixed are loaded. The charginginlet is sealed with a lid and seal arrangement, which provides an airtight seal to the mixing vessel 140 on closure of the lid, to enable thevessel to be pressurised.

In the illustrated embodiment, the loading of at least some of theconstituent materials (in this case sand, water and cement to makeconcrete), is achieved automatically without manual loading using asuction conduit assembly 160 substantially as described above inrelation to FIGS. 1 to 3A (equivalent parts are designated by the samereference numerals). Additional loading means include a cement hopper180 and a water supply 190 which are also in communication with themixing vessel 140 via the charging inlet.

A discharge outlet 200 is provided in the base of the mixing chamber140, to which is connected a delivery hose 210. The delivery hose 210transports the discharged mixture and conveys it by pneumatic pressure,supplied via supply line 220 from the compressor 120, when the dischargevalve is opened.

In use, the engine 110 is started, and the controls are set to operatethe jet pump 62. The engine also drives the mixing paddles in the mixingvessel 140, which rotate to agitate and mix the added constituents.

The mixing vessel 140 is evacuated, and the sand is added to the mixingvessel via the suction conduit assembly 160. At the same time, the valveto the cement hopper 180 is opened to admit a pre-determined amount ofcement to the mixing vessel, and similarly a volume of water from thewater supply 190 is also emptied into the mixing vessel. The valves tothe cement hopper 180 and water supply 190 are closed once the requiredamount has been discharged into the vessel 140.

Once the constituents are sufficiently mixed to the requiredconsistency, the mixing vessel is pressurised using air from thecompressor 120 along supply line 220. It can be seen thatadvantageously, the same air supply may be used to supply both the jetpump 62 and to pressurise the mixing vessel 140, as each requires theair supply at a different stage in the process. The valve in thedischarge hose 210 is opened and the mixture is discharged via thedischarge outlet 200 and is conveyed along discharge hose 210 to itsrequired location. A conventional tripod arrangement 220 may be providedto kill the energy in the mixture (concrete) and allow it to fall in aheap to be spread.

As can be seen in FIG. 5, the mixing vessel 140 is generally ofspherical configuration. The pressure shell 141 forming the outer partof the mixing vessel 140 is formed from a pair of hemispheres producedby pressing or spinning. The shell is provided with 12 lining segments142. These sufficiently duplicate the form of the pressure shell 141,which they protect from abrasive wear of the constituents to be mixedwithin the vessel. Eight of the lining segments 142 are reversible, topto bottom. The charging inlet 143 is provided at the top of the vessel140, and the discharge outlet 200 is provided slightly offset from thebottom of the vessel 140.

As can be seen in FIGS. 6 a and 6 b, the charging inlet of the mixingvessel 140 is provided with a lid 210 to seal the vessel. Water isadmitted by a branch formed in the lid housing 211. It passes through ahole in a flat seal 212 and thence to a nozzle formed in the sealretaining place 213. A non-return valve (not shown) prevents reverseflow through the branch when the vessel 140 is pressurised. The sealplate 213 and the housing 211 are formed to dish the seal.

The shape of the seal plate 213 and housing 211 forms the seal shape,thus allowing a low cost flat seal to be used. The dished form of theseal 212 provides a simple means of giving a low-torque sealing actionwhen under vacuum and a tight seal when under pressure.

As can be seen in FIGS. 7 a and 7 b, the water provided to the mixingvessel 140 can be used to form a washing jet. The energy to drive thewashing jet is provided by the vacuum generated in the mixing vessel 140by the jet pump 62.

The washing jet may be used to provide a directional water injection toclean an internal screen area. All constituent material enters themixing vessel 140 via a circular screen 215, which is located beneaththe lid 210. The screen 215 is crowned, with radial and circumferentialvanes, and serves to trap material such as fragments of bag and stoneswhich should not be present in the sand, or stones, but which have beensucked up from beneath the sand heap. A gully surrounds the screen.Water is injected into the screen area through a nozzle 216 which isadjustable in orientation, for example the nozzle may be rotatable todirect the water spray as required.

The conical screen form is largely self-cleaning, with stones which havebeen sucked up falling to the gully surrounding the screen. Cleaning isassisted by the water jets. The adjustability of the nozzle 216 allowsit to be set effectively. Advantageously, the washing water forms partof the concrete mix.

As can be seen in FIGS. 8 a to 8 h, there are two identical securinglatching assemblies 217 for the lid assembly 210 for the mixing vessel140. When the mixing vessel 140 is pressurised, opening is prevented bythe internal pressure acting through over-centre links, lever 218 andhandle extension 219, and a cam, part of shaft 220, arrangement workingthrough a small lever, part of shaft 220 shown by Section 8 f-8 f (FIG.8 f).

The travel of this lever is controlled by the flat bar, part of shaft220, shown on 8 h-8 h (FIG. 8 h) and two roll pins 221. The shaft 220 isheld in the closed position by a torsion spring 222. Additionalresistance to rotation is introduced by valve 223 shown in FIG. 9. Thisis held closed by the pressure in the vessel 140 plus the spring load ofitem 220 required to prevent the valve 223 being opened by a vacuumwhile the vessel 140 is filling.

Adjustment of the valve 223 is by means of a screw 224 prevented fromloosening by a self-locking helical insert 225. Valves must be left openbefore starting the engine 110 to prevent overload of the compressor120. The engine 110 cannot be started until both latch levers 218 havesecured the lid 210. Up to this point the limit switch 226 on Section 8f-8 f de-energises the solenoid shut-down valve on the fuel pump. Thelimit switch 226 under the lid 210 shown on Section 8 e-8 e (FIG. 8 e)also activates the solenoid shut-down valve.

To prevent accidental shut-down, the pressure in the mixing vessel 140must be released by means of a gate valve (not shown) before it ispossible to operate the small lever, part of shaft 220, in turnoperating the limit switch 226 and venting any residual pressure throughbranch 227, part of the pressure vessel 140 and release valve 223. Themain lever 218 cannot be rotated until the cam, part of shaft 220, onSection 8 g-8 g (FIG. 8 g) is rotated by means of the small lever, partof shaft 220, shown on Section 8 f-8 f (FIG. 8 f).

If at any time the lid 210 is raised, perhaps through failure of part ofthe mechanism described, the jet emerging from the gap between lid 210and vessel 140 is deflected downwards by the profile of the edge of thelid. It is unlikely that failures of both latch mechanisms will failtogether.

As can be seen in FIGS. 10 a and 10 b, a modification to standard ballvalve for the suction pipe 160 is provided. A ball valve will admitsolids into the cavity between the seals, if it is operated when notcompletely clean. A standard ball valve will then quickly jam, and isthus not usable in the mixer-placer environment. In the presentinvention, the body of a conventional ball valve 228 is drilled throughon a diameter. A steel split cover 229 is formed with branches tocorrespond with these ports when clamped into position. A seal 230 isformed in-situ by a two-part sealing compound. Once every operatingcycle, when the vessel 140 is under vacuum, water is flushed through thevalve via the branches in the cover 229.

This modification makes a standard, low-cost ball valve suitable forvacuum solids transmission. The energy to drive the sluicing water isprovided by the vacuum generated in the vessel 140 by the jet pump 62.Advantageously, the sluicing water forms part of the concrete mix. In analternative embodiment (not shown) the ball valve is replaced with apinch valve.

FIG. 11 illustrates the valve assembly provided in the discharge hose210. It has been found advantageous to inject air into the flowdischarged from the mixing vessel 140 in order to achieve discharge tothe required distance and elevation from the available pressure. Thereis provided a duckbill chopper valve 239 (so called because it chops upwhat would otherwise be a solid column of concrete mix into alternatingslugs of air and mix) through which air is admitted. The valve element231 is formed from reinforced rubber. A branch is provided in thehousing 232 for water cleaning if necessary.

The lined duckbill form needs no maintenance. The greater the pressure,the better the seal from a thick, soft natural rubber lining. The formis also strictly one-way and concrete cannot be forced into the airline.

As can be seen in FIG. 12, there is provided a safety mechanism, in theevent that one of the mixing paddles in the mixing chamber 140 shouldjam. In this event, a torque limiter prevents damage to the gearbox. Thetorque limiter operates by shearing a sacrificial shear pin 233. Whenthe pin 233 shears, a spring 234 pushes the shear pin carrier 235,located in a fork extension on the drive shaft 236, away from the driveshaft 236 and operates a limit switch 237 which in turn shuts the engine110 down via the fuel pump solenoid. The ends of the sheared pin wouldtend to fly out, but are retained by the screws attaching shear pinhousing 238. The waist of the shear pin is located in an annular gapbetween shear pin carrier 235 and shear pin housing 238.

In this way, damage to the gear box and remainder of paddle drive traincaused by jamming is prevented. The engine 110 is shut down immediatelythe shear pin fails, and the broken shear pins cannot escape untilreleased. The annular gap prevents internal damage which might be causedby the ends of the shear pin fragments.

FIGS. 13 and 14 illustrated a suction conduit assembly according to afurther embodiment of the invention. In this embodiment, the suctionconduit 260 is in the form of an articulated arm assembly. Other thanset out below, the functioning of the suction conduit is the same ashereinbefore described.

The articulated suction inlet pipe 260 is part balanced by gas springs262. A spring clip 264 locates in grooves in hose 266 extension tubecarrying a seal to prevent loss of vacuum. The suction pipe isarticulated at the centre about a horizontal axis, in gimbalarrangement. The mean weight of the pipe and contents is balanced by gassprings. The end of the suction pipe carries a vibrator 268.

It has been found that with this arrangement, the assembly of thesuction pipe to the pressure vessel 141 is quicker. The articulation ofthe suction pipe allows access to a large volume of sand without movingthe machine. The vibrator 268 secures continuous collapse of the sandheap, securing a constant feed to the pressure vessel and minimum cycletime.

1. Aggregate mixing apparatus comprising: a substantially sphericalbatch mixing vessel having an inlet to receive constituent solidaggregate material, an agitation assembly for mixing the solid aggregatematerial within the vessel and an outlet to convey batches of mixedmaterial, a pressurizing device for pressurizing the mixing vessel toassist in evacuation of the mixed material from the outlet, a batchloader including: a conduit through which the solid aggregate materialare conveyed in use by fluid flow into the batch missing vessel, and ajet pump to create a vacuum to load the mixing vessel with the solidaggregate material, without the need for manual loading, a singlepneumatic source used both to pressurize the mixing vessel and to drivethe jet pump.
 2. Apparatus according to claim 1 wherein the mixingvessel is provided with a clamping arrangement for preventing accidentallifting of a lid when the vessel is under vacuum.
 3. Apparatus accordingto claim 1 wherein the batch loader comprises an inlet suction conduitassembly which includes an adjustment device, such that the effectivelength of the conduit assembly can be varied in use.
 4. Apparatusaccording to claim 3 wherein the conduit assembly comprises a sectionwhich is capable of telescopic movement.
 5. Apparatus according to claim1 wherein the conduit assembly is provided with an arrangement operableto vibrate the intake.
 6. Apparatus according to claim 1 wherein anairflow is provided in the flow discharged from the mixing result. 7.Apparatus according to claim 1 wherein the mixing vessel is providedwith a level sensor.
 8. Apparatus according to claim 7 wherein the levelsensor is an oscillating-vane level sensor.
 9. Apparatus according toclaim 1 wherein the agitation assembly comprises mixing paddles. 10.Apparatus according to claim 1 wherein the agitation assembly ishydraulically driven.